On this day, April the 20th, 1999...
Two teenage gunmen kill 13 people in a shooting spree at Columbine High School in Littleton, Colorado. At about 11:20 a.m., Dylan Klebold and Eric Harris, dressed in long trench coats, began shooting students outside the school before moving inside to continue their rampage. By the time SWAT team officers finally entered the school at about 3:00 p.m., Klebold and Harris had killed 12 fellow students and a teacher, and had wounded another 23 people. Then they turned their guns on themselves and committed suicide.
Kids their age don't plan ahead that far and rarely agree on anything, so there had to be an adult, a sick adult, that supplied the explosives, guns, plans, and double suicide -- Hypnosis or brainwashing is a common practice now, and in light of the Jones Massacre and Heavens Gate suicides, that showed how susceptible even the more intelligent are, I was surprised the police didn't consider it at the time.